Engineering Tunable Single and Dual Optical Emission from Ru(II

Article pubs.acs.org/JPCC

Engineering Tunable Single and Dual Optical Emission from Ru(II)− Polypyridyl Complexes through Excited State Design Julia Romanova,† Yousif Sadik,† M. R. Ranga Prabhath,†,§ J. David Carey,†,‡ and Peter D. Jarowski*,† †

Advanced Technology Institute and ‡Department of Electrical and Electronic Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom S Supporting Information *

ABSTRACT: Excited state design is an efficient approach toward new applications in molecular electronics spanning solar cells, artificial photosynthesis and biomedical diagnostics. Ruthenium(II)−polypiridyl based complexes are an example of molecular building blocks with tunable single and dual wavelength emission that can be controlled by excited state engineering via selective ligand modification. Here we investigate three new heteroleptic [Ru(bpy)2X]+ complex ions, where X represents pyridinyl or pyrazinyl derivatives of diazolates, providing tunable emission in the visible and infrared region. The dual emission is shown to arise from the presence of two excited states consisting of a triplet metal-to-ligand charge transfer state localized on a bipyridine ligand, 3MLCT (bpy), and a state that either is entirely localized on the X ligand or is partially delocalized also spanning part of the bipyridine ligands, 3MLCT(X). By a suitable choice of the X ligand, emission from 3MLCT(bpy) and 3MLCT (X) states can be rationally varied between 743 and 865 nm and from 555 to 679 nm, respectively. An increase in the nitrogen content of the sixmembered ring of the X ligand results in a blue shift of the 3MLCT(bpy) emission but a red shift for the 3MLCT (X) emission. The wavelength difference between 3MLCT(bpy) and 3MLCT (X) emissions can be tuned from 84 to 310 nm and is proportional to the difference in LUMO energies (reduction potentials) of the isolated ligands. Our study provides key information toward new routes for the design of optically active dual wavelength molecular emitters.

1. INTRODUCTION Advances in molecular design have led to the synthesis of new functional molecules with tailored properties and to the successful generation of supramolecular architectures. Being able to predict the structural and electronic properties of molecules through quantum mechanical calculations is a key tool in modern molecular science. To date most successes using density functional theory (DFT) calculations have been centered on the determination of ground state molecular structure and properties, as well as on the identification and characterization of frontier molecular orbitals.1 Recent advances in the optimization of excited state geometries and efficient computational algorithms have seen a renewed focus on the use of quantum mechanical calculations as a tool to design molecular excited state properties and associated optical emission.2 Ru−polypyridyl based complexes represent one such class of compounds to which excited state molecular design is particularly worthy of application.3 These complexes possess intriguing excited state properties and are powerful structural motifs4 in the design of advanced optical materials for dye sensitized solar cells,5−7 light-emitting electrochemical cells,8 light-emitting diodes,9 and nonlinear optics.10 Furthermore, Ru−polypyridyl complexes have also been used as antitumor agents,11,12 in artificial photosynthesis,13 and as molecular switches and machines.14−17 The wide range applications of Ru−polypyridyl complexes invokes an enormous amount of fundamental and experimental studies and the © XXXX American Chemical Society

interest in the optical properties of these compounds still continues to increase.18−26 The further design and control of optical materials based on Ru−polypyridyl will require an explanation and predictive prescription for, perhaps, their most intriguing optical aspect: the observation of single and dual optical emission.27−38 In general, dual emission properties in organometallic complexes may arise from 3MLCT states associated with two different ligands or with two different metals in dissymmetric bimetallics complexes, as well as from two different in character excited states (3MLCT and 3IL; 3MLCT and LL′CT). To be a dual emitter, the mononuclear Ru-polypyridyl complex should be heteroleptic, i.e., one of its bipyridine (bpy) coordination sites should be replaced by an alternative ligand (X). Such heteroleptic nature implies, therefore, the existence of two metal-to-ligand charge transfer excited states with triplet character (3MLCT). Due to the three-pronged propeller shape of Ru-complex ions, it is possible that these two 3 MLCT excitations are predominantly localized on individual ligands: one mainly associated with the bpy coordination site, and the other spanning the X ligand. If the electronic coupling between such spatially isolated 3MLCT states is weak, dual emission from Ru-complex ions can occur (Scheme 1).30,31 On Received: October 11, 2016 Revised: December 1, 2016 Published: January 1, 2017 A

DOI: 10.1021/acs.jpcc.6b10263 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. Model30,31 Explaining Low-Temperature (120−220 K) Dual Emission Properties of the Heteroleptic Complex Ion of Type [Ru(bpy)2X]+ a

a

The radiative relaxation processes are denoted by solid lines, and the nonradiative ones, with dashed lines.

the contrary, strong electronic coupling would evolve from delocalized 3MLCT excited states over all ligands. Such delocalization would ensure a relatively fast thermal equilibration of the different 3MLCT states and single emission from the lowest in energy 3MLCT state, i.e., classical Kasha’s behavior.39 It has also been demonstrated that single40 and dual31 emission depend on the energy of the metal-centered triplet excited state (3MC), which is a dark state and is regarded as the main photodeactivation channel.41 In a weak ligand field the 3MC excited state is stabilized and if its energy is lower than the 3 MLCT excited states, fast radiationless decay occurs. Alternatively, it is expected that in a strong ligand field the 3 MC excited state is destabilized with respect to the 3MLCT states. In this case single/dual emission can be detected and depending on the extent of the destabilization there is a possibility for thermal equilibration30,31 between the 3MC state and the 3MLCT states (Scheme 1). Due to the complex photophysics in Ru−polypyridyl complexes, the origin of their dual emission properties is still debatable and the nature of their excited state is not fully understood thereby lending an oportunity for detailed excited excited state computations.32 The present study is dedicated to the understanding of the potential single and dual emission properties of Ru−polypyridyl complex ions. In particular, we focus on six heteroleptic complex ions of type [Ru(bpy)2X]+, where X denotes anionic ligand: 2-pyridyl or 2-pyrazine derivatives of 1,2-diazolate, 1,3diazolate, and 1,2,4-triazolate (Scheme 2). The series of six complex ions have been selectively chosen so that the nitrogen content and the nitrogen position in the X ligand can be varied systematically. We have shown in our laboratories that systematic efforts to modify structural aspects of optical

Scheme 2. Chemical Structure of the Investigated [Ru(bpy)2X]+ Complex Ions and Ligands and Their Abbreviationsa

a

The coordination sites of the X ligands are denoted with arrows. To distinguish between the two bpy ligands, they are denoted as L (left) and R (right) depending on their location with respect to the X ligand.

B


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Table 1. Calculated Wavelengths [nm] for Vertical S0 → Tn Excitation/Emission Tn → S0 Transitions in [Ru(bpy)2X]+ Complex Ionsa 12PYAZ

13PYAZ

124PYAZ

12PZAZ

(d−πazo)−πbpy * 545/780 T1 (d−πazo)−πbpyL* 536/NA T2 (d−πazo)−πazo* 440/NA T4

13PZAZ

124PZAZ

R

T1

576/865

T1

565/850

T1

T2

560/749

T2

553/NA

T2

T7

436/555

T12

387/NA

T7

555/819

T1

546/808

T1

526/743

540/NA

T2

534/NA

T2

517/NA

T8

441/NA

T3

501/659

499/679

πbpyR*,

The results are obtained in acetonitrile by the TD-DFT/PBE0 approach. In the table (d−πazo), and πazo* indicate the character of the molecular orbitals involved in the electronic transitions, and their notation is explained in the text. NA indicates root-flipping problems during excited state optimization. a

πbpyL*,

shows that the optimized structures represent true minima on the potential energy surface. Vertical electronic excitations and excited state optimizations were obtained by TD-DFT59,60 with the same functional, basis sets and solvation model as for ground state geometry optimization. All quantum-chemical calculations were performed with the Gaussian 09 program package.61 The molecular orbitals were visualized with an isosurface value of 0.04 au by using the GaussView 5 graphical interface.62

materials yield important design criteria and here we apply this strategy to excited state engineering in Ru−polypyridyl complex ions.42−47 Three of these complex ions, 12PYAZ, 12PZAZ, and 13PZAZ, are computationally designed in our laboratory and have not been reported previously. The other three complex ions that form part of this study, 13PYAZ, 124PYAZ, and 124PZAZ, are already known in the literature,30,31,48−52 which helps to benchmark our calculations as well as to provide further insight into the emission mechanism. It has been experimentally demonstrated that 13PYAZ and 124PYAZ possess single emission properties in solution, whereas 124PZAZ has been confirmed as a single emitter at room temperature (620 nm) and dual emitter at low temperature (590 and 700 nm). The dual emission properties of 124PZAZ are described in Scheme 1, which helps to explain the temperature dependence of the second emission band;30,31 therefore, the 124PZAZ complex ion serves as a reference compound in our study. The present theoretical investigation aims to (1) explore how the nitrogen content and the nitrogen position in the X ligand affects the energy and the properties of the first and the second potentially emitting 3MLCT excited states; (2) to reveal the electronic structure of the 3MLCT excitations and to estimate the extent to which they are localized (weakly electronically coupled) or delocalized (strong electronically coupled); (3) to provide guidelines for tuning of the single and dual emission properties in heteroleptic Ru−polypyridyl complexes that can be employed during chemical synthesis. Engineering 3MLCT excited states is a key to control the single and dual emission properties of Ru−polypyridyl complex ions, and we show here that such control can be achieved by chemical modification of the ligands. The results are obtained by the density functional theory (DFT) and its time-dependent (TD) formulation in combination with PBE0 functional and a polarizable continuum model (PCM) of the solvent. The same computational strategy was recently applied in our laboratory with great effect in the design of platinum complexes with tunable luminescence properties.42−44

3. RESULTS AND DISCUSSION The optimized ground state geometries of all complex ions are represented in Figure S1, Supporting Information. Comparison between the calculated Ru−N bond lengths and N−Ru−N valence angles for 124PYAZ and 124PZAZ with experimentally available X-ray measurements on similar compounds49,63 can be found in Figure S1, Table S6, and Table S7 (Supporting Information). Our results reveal good agreement between the experimentally observed ground state geometries and the ones predicted by the PBE0 functional. The lowest in energy 1 MLCT transitions for the three known complex ions are calculated at 519 nm (13PYAZ), 509 nm (124PYAZ), and 492 nm (124PZAZ). Their experimentally recorded absorption spectra reveal maxima with 1MLCT origin at 490 nm (13PYAZ),51 474 (124PYAZ),48 and 456 nm (124PZAZ).52 The mean average error for the excitation energies is about 0.17 eV and this result additionally supports the choice of the PBE0 functional for investigation of structure−property relationships in the Ru−polypyridyl ions. 3.1. Identification of the 3MLCT Excited States. The first step in our investigation was to identify all key 3MLCT states (Scheme 1) at the ground state minima. Results for the key S0 → Tn transitions are represented in Table 1 and detailed information on all calculated triplet excitations can be found in the Supporting Information, Table S1. S0 → Tn Electronic Transitions with (d−πazo)−πbpy* Character. Our results indicate that each complex ion is characterized with S0 → T1 and S0 → T2 transitions, which are very close in energy (Table 1). The S0 → T1/S0 → T2 excitations are calculated at 576/560 nm (12PYAZ), 565/553 nm (13PYAZ), 545/536 (124PYAZ), 555/540 nm (12PZAZ), and 546/534 nm (13PZAZ) and 526/517 nm (124PZAZ). The analysis of the molecular orbitals (MOs) reveals that the lowest in energy triplet excited states involve HOMO → LUMO (T2) and HOMO → LUMO+1 (T1) electronic transitions (Figure 1). In all cases the HOMO is found to be localized on the ruthenium ion and the azolate ring and is denoted as d−πazo orbital. By contrast, LUMO and LUMO+1 densities span the L and R bipyridine ligands (Scheme 2), denoted as πbpyL* and

2. COMPUTATIONAL METHODS Ground state geometry optimization and vibrational frequencies analysis of the [Ru(bpy)2X]+ complex ions and the isolated ligands were performed with the PBE0 functional,53 the Stuttgart/Dresden (SDD) pseudopotential and basis set for Ru,54 and the Pople 6-31G* split-valence basis set55 for the ligands. The polarizable continuum model (PCM)56−58 in its integral equation formalism variant was applied to take into account the electrostatic interaction with the acetonitrile solvent (ε0 = 35.69, ε∞ = 1.81). The vibrational analysis C


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consistent with the stronger π-accepting properties (lower reduction potential) of the isolated bipyridine ligand with respect to the isolated X ligand (Supporting Information, Table S3). As a result, the electronic transition from HOMO (d−πazo) to LUMO+2 or to LUMO+3 (πazo*) represent excited states above T2. The triplet transition involving πazo* as an accepting orbital is calculated to be at 436 nm (12PYAZ), 387 nm (13PYAZ), 440 nm (124PYAZ), 499 nm (12PZAZ), 441 nm (13PZAZ), and 501 nm (124PZAZ), respectively (Table 1). 3.2. Structure of T1 Excited States with (d−πazo)−πbpyR* Character. The single emission properties of the complex ions are expected to originate from T1, the 3 MLCT excited state with (d−πazo)−πbpyR* character. The optimized geometries of these excited states are represented in Figure 2. Despite the different electronic structure of the X ligand, all T1 states show similar structural transformation with respect to S0 states (Supporting Information, Figure S1): (1) the Ru−N bonds in the L bpy ligand increases and the geometry of the L bpy ligand remains aromatic; (2) one of the Ru−N bonds in the R bpy ligand decreases, and the geometry of the R bpy ligand transforms to quinoid; (3) the Ru−N bond in the azolate cycle decreases, but elongation is observed for the Ru−N bond in the pyridine cycle of the X ligand; and (4) the conjugation pattern within the azole cycles changes leading to an elongation of the interring bond in the X ligands. The strength of the interactions between the metal ion and the azolate cycle in T1 varies as a function of the X ligand. Proceeding from S0 to T1 the Ru−N bond for the azolate ring decreases by 0.061/0.057 Å in 124PYAZ/124PZAZ, by 0.077/ 0.074 Å in the 12PYAZ/12PZAZ, and by 0.078/0.075 Å in 13PYAZ/13PZAZ derivatives. This trend correlates with the HOMO energies (the π-donor properties) of the isolated X ligands (Supporting Information, Table S3). When the HOMO energy of the X ligand increases, the decrease in the Ru−N bond upon excitation is stronger and the structural relaxation with respect to the X ligand becomes more pronounced. On the contrary, the quinoidization of the R bpy ligand is similar in strength in all T1 of the complex ions. 3. 3. T 1 → S 0 E l e c t r o n i c T r a n s i t i o n s w i t h (d−πazo)−πbpyR* Character and Single Emission Properties. The calculated emission energies are also listed in Table 1. The dominant electronic configuration in all T1 states involves HOMO (d−πazo) and LUMO (πbpyR*) (Figure 3). The T1 → S0 transitions are theoretically predicted at 865 nm (12PYAZ), 850 nm (13PYAZ), 780 nm (124PYAZ), 819 nm (12PZAZ), 808 nm (13PZAZ), and 743 nm (124PZAZ). Therefore, it can be concluded that a blue shift in the first emission wavelength is observed when the nitrogen content in the six-membered ring of the X ligand increases. In addition, the blue shift in the first emission wavelength also increases as 1,2- > 1,3- > 1,2,4derivatives. The observed tunability originates from the variation of the ground state HOMO (d−πazo) energies of the complex ions, rather than in the ground state LUMOs (πbpyR*) energies (Supporting Information, Table S4), i.e., from the different Ru-X ligand interaction (see ground state Ru-X bonds, Supporting Information, Figure S1). 3.4. Structure of T2 Excited States with (d−πazo)−πbpyL* Character. Our calculations indicate that the lowest in energy triplet excited state of the complex ions originates from a (d−πazo)−πbpyR* transition and that such excitation can be associated with the lowest in energy emission band. This is in agreement with resonance Raman measurements on 124PZAZ demonstrating that the vibronic coupling

Figure 1. Leading configurations for the key electronic transitions in 12PYAZ: (A) S0 → T1, (B) S0 → T2, and (C) S0 → T7. The leading electronic configurations for the rest of the complex ions are analogous. Detailed information on the CI coefficients and MO representations for all complex ions can be found in the Supporting Information, Table S2 and Figure S2.

πbpyR*, respectively. The small energy difference between the T1 and T2 states originates from the stronger interaction of Ru(II) with the R bipyridine ligand than with the L bipyridine ligand in S0 (Supporting Information, Figure S1), i.e., from the πbpyL*/ πbpyR* energy splitting. S0 → Tn Electronic Transitions with (d−πazo)−πazo* Character. The heteroleptic nature of the complex ions implies the existence of charge transfer excitation to an accepting molecular orbital of the X ligand (πazo*). The MO results show that in the series of complex ions, LUMO+2 or LUMO+3 represents the first πazo* orbital (Figure 1). It is important to note that the πazo* orbital has a higher energy than either the πbpyL* and πbpyR* orbitals. Such energy level alignment is D


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Figure 2. Optimized bond lengths [Å] in T1 excited state of [Ru(bpy)2X]+ complex ions.

in the first excited state causes the appearance of characteristic signatures for reduced bipyridine ligand (bpy•−).30,31 Our results suggest that a candidate for a higher energy emitting state is T2, which possess (d−πazo)−πbpyL* character. To our knowledge there is no experimental evidence supporting that a triplet state with (d−πazo)−πbpyL* character is associated with the second emission band in [Ru(bpy)2X]+ complex ions. Moreover, according to resonance Raman investigations on

124PZAZ, the second emission band is associated with a structural relaxation within an X ligand.30,31 However, even if T2 is not associated with the second emission band, its presence can affect the excited state photodynamics in [Ru(bpy)2X]+ complex ions because T 1 , (d−π azo )−π bpy R * and T 2 , (d−πazo)−πbpyL* are relatively close in energy. The splitting is about 0.22 eV (Table 1); it is therefore interesting to compare the electronic structures of these low in energy triplet states. E


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configurations for T1 and T2 in the excited state minima. In the T2 state the d-AO from the HOMO and the pz-AOs from the LUMO are orthogonal (see Figure 3, T2, 12PYAZ, HOMO top vs LUMO side), whereas in the T1 state the d-AO from the HOMO is coplanar to the pz-AOs in the LUMO (see Figure 3, T1, 12PYAZ, HOMO top vs LUMO side). Therefore, our calculations imply a stronger metal−azolate interaction in the T1 excited state than in the T2 one. This also suggests the existence of better electronic communication between (d−πazo)−πbpyR* and (d−πazo)−πazo* excited states rather than between (d−π a z o )−π b p y L */(d−π a zo )−π a z o * and (d−πazo)−πbpyR*/(d−πazo)−πbpyL* excited states. 3.5. Structure of Higher Excited Triplet States with (d−πazo)−πazo* Character. Another candidate for the second emitting excited state is the triplet with (d−πazo)−πazo* character. There is experimental evidence for the 124PZAZ complex ion,30,31 which supports this hypothesis and shows that the second emission band is associated with a structural relaxation within the X ligand. Therefore, it is important to understand the molecular geometry and properties of the (d−πazo)−πazo* triplet excited state. Due to the root-flipping problems, we were able to optimize three of the six excited triplet states with (d−πazo)−πazo* character. In particular, they are T7 (12PYAZ), T4 (12PZAZ), and T3 (124PZAZ) and their optimized structures are represented in Figure 4. Our results reveal that upon excitation from S0 to (d−πazo)−πazo* triplet state the interaction of the metal with the X ligand and ruthenium increases (Supporting Information, Figure S1). Such strong interaction is apparent from the very short Ru−N bonds for the X ligand calculated for these excited states. On the contrary, the interaction with the bipyridine ligands decreases. As a result, the geometry of the bipyridine ligands remains aromatic, while the conjugation pattern of the X ligand changes. This change is stronger in the case of 12PYAZ and 124PZAZ than in the case of 12PZAZ. For example, with respect to ground state the inter-ring bond length in 12PYAZ and 124PZAZ decreases by 0.35 and 0.34 Å, respectively, whereas in 12PZAZ it decreases by 0.005 Å. An explanation for this difference can be found from the MO analysis (Figure 3, Figure 5, and Figure 6). As can be seen, the coefficients on the azolate ring in the accepting molecular orbitals are almost zero in the case of 12PZAZ, showing a weaker contribution of the fivemembered in the excited state structural relaxation. 3.6. Tn → S0 Electronic Transitions with (d−πazo)−πazo* Character. The calculated emission energies are listed in Table 1. The second emission wavelengths are calculated to be at 555 nm (12PYAZ), 679 nm (12PZAZ), and 659 nm (124PZAZ). Therefore, it can be concluded that an increase of the nitrogen content in the six-membered ring of the X ligand results in a red shift in second emission wavelength. The red shift is also apparent from the S0 → Tn calculations (Table 1). In addition, the S0 → Tn results indicates that a red shift in the second emission wavelength can be obtained by using 1,2- and 1,2,4derivatives rather than 1,3-derivatives. The observed tunability originates from the variation of the ground state HOMO (d−πazo) energies of the complex ions, as well as from their ground state LUMO+2/+3 (πazo*) energies (Supporting Information, Table S4). Therefore, the tunability in the second 3 MLCT emission are related to the donor and acceptor properties of the isolated X ligands (Supporting Information, Table S3). The MO analysis also shows that in 12PYAZ complex ions the (d−πazo)−πazo* excitation is strongly localized on the X ligand. Therefore, our theoretical results suggest that

Figure 3. Leading electronic configurations for 3MLCT excited states of 12PYAZ: (A) T1, (B) T2, and (C) T7. The leading electronic configurations for T1 of the rest of the complex ions are analogous. Detailed information on the CI coefficients and MO representations for all complex ions can be found in the Supporting Information, Table S5 and Figure S3.

Due to root-flipping problems we were able to optimize T2, MLCT only in the case of 12PYAZ and its structure is shown in Figure 4. The MO analysis indicates that the leading MOs in this excited state are HOMO and LUMO+1 (Figure 3) A comparison between T1 and T2 excited states of 12PYAZ reveals that upon excitation to T1 the R bpy ligand undergoes an aromatic-to-quinoid deformation, whereas in T2 the L bipyridine is subject to quinoidization. An important difference between T1 and T2 is the metal−azolate interaction. In the T2 state the metal−azolate interaction is weaker than in T1. This could be explained by looking at the leading electronic 3

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Figure 4. Optimized bond lengths [Å] in higher excited triplet states of [Ru(bpy)2X]+ complex ions. The results are obtained in acetonitrile with the TD-DFT/PBE0 approach.

only in the case of 12PYAZ complex ion the (d−πazo)−πbpyR* and (d−πazo)−πazo* excited states can be regarded as entirely spatially isolated. By contrast, in 12PZAZ and 124PZAZ complex ions the (d−πazo)−πazo* excitation is mainly associated with the X ligand but spans also the bipyridine ligands; i.e., it is delocalized to some extent. Such delocalization suggest an increase in the probability for electronic coupling between the (d−πazo)−πbpyR* and (d−πazo)−πazo* triplet excited states in 12PZAZ and 124PZAZ complex ions. The mixing between the accepting ligand orbitals, as well as their delocalization increases when the energy difference between S0 → Tn transitions with (d−πazo)−πbpyR* and (d−πazo)−πazo* character decreases (Table 1). These results are also in line with the energy difference between the LUMOs (reduction potentials) of the isolated X and bipyridine ligands (Supporting Information, Table S3). When the energy difference between the LUMOs of the isolated X and bipyridine ligands decreases the probability for electronic coupling between the (d−πazo)−πbpyR* and (d−πazo)−πazo* triplet excited states is expected to increase. And if this energy difference is small enough, this will cause deactivation of the (d−πazo)−πazo* excitation and to the lack of second emission peak. 3.7. Energy Difference between Excited Triplet States with (d−πazo)−πbpyR* and (d−πazo)−πazo* Character and Dual Emission Properties. Regarding the dual emission properties it is important to evaluate the energy difference between the two possible emission bands originating from T1

with (d−πazo)−πbpyR* character and Tn (n > 2) with (d−πazo)−πazo* character. This can be done by using the vertical Tn → S0 emission energies/wavelengths (Table 1) and will be referred here as the direct approach. Our results indicate that the energy difference (wavelength difference) obtained by the direct approach amounts to 0.80 eV (310 nm) in 12PYAZ, 0.31 eV (140 nm) in 12PZAZ, and only to 0.21 eV (84 nm) in 124PZAZ. This suggests that the chemical modification of the X ligand can be used to tune the energy difference between the first and second emission bands. There is also an alternative and much computationally cheaper TD-DFT approach to estimate the energy difference between the (d−πazo)−πbpyR* and the (d−πazo)−πazo* excited states. In this alternative approach the energy difference can be approximated by using the vertical excitation S0 → Tn energies/wavelengths (Table 1) instead of the emission ones. With the alternative approach the energy difference (wavelength difference) is estimated as 0.69 eV (140 nm) in 12PYAZ, 1.01 eV (187 nm) in 13PYAZ, 0.54 eV (105 nm) in 124PYAZ, 0.25 eV (56 nm) in 12PZAZ, 0.54 eV (105 nm) in 13PZAZ, and 0.12 eV (25 nm) in 124PZAZ. It is apparent that the alternative approach underestimates the energy difference but gives qualitatively similar results to the case where the energies of the excited state minima are used. Therefore, on the basis of the qualitative agreement between the direct and the alternative approach, we can suggest that the energy difference between the peaks of the two emissions becomes larger when the nitrogen content in the six-membered G


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Figure 5. Leading electronic configurations in the T4 3MLCT excited state of 12PZAZ. Detailed information on the CI coefficients can be found in the Supporting Information, Table S5.

ring of the X ligand and when going from 1,2,4- < 1,2- < 1,3azolate derivatives. These tunability trends are in line with the energy difference between the LUMO levels (reduction potentials) of the isolated ligands (Supporting Information, Table S3). It is important to note that for 124PZAZ the experimentally measured emission maxima30,31 at low temperature are at 590 and 700 nm. For comparison, at room temperature single emission maximum for 124PZAZ is located at 660 nm. Our calculations (Table 1) reveal vertical emission wavelengths (estimated from the difference between the S0 and Tn energies at Tn geometry) at 659 nm (T3 → S0) and 743 nm (T1 → S0). The calculated adiabatic emission wavelengths (estimated from the difference between the S0 and Tn energies at their respective geometry) are estimated at 567 nm (T3 → S0) and 611 (T1 → S0) nm. Therefore, it can be concluded that there is a good agreement between the calculations and low-temperature measurements on the dual emission properties of 124PZAZ complex ions. The quantitative differences between the calculated and experimentally measured dual emission maxima can be explained by the lack of counterions in our molecular models (for example BF4−), which can stabilize the excited states with respect to the ground state and by the fact that the vibronic structure of the emission bands and temperature effects are not taken into account in the theoretical simulations. 3.8. 3MC Dark State as a Photodeactivation Channel. Single and dual emission from a 3MLCT excited states will

Figure 6. Leading electronic configurations in the T3 3MLCT excited states of 124PZAZ. Detailed information on the CI coefficients can be found in the Supporting Information, Table S5.

occur if the dark 3MC state, which is a main photodeactivation channel, lies relatively high in energy.41 Because we were not able to optimize the 3MC states (although we have tried several possible initial geometry guess and different approaches), here we will present a brief discussion on the qualitative aspects associated with the dark states and the X ligands chemical composition. One can expect that the energy of the 3MC state can be tuned by X ligand modification and that the destabilization of the dark state should be in line with the πdonor properties/HOMO energies of the isolated ligands. Taking into account the π-donor properties/HOMO energies of the X ligands (Table S3, Supporting Information) the H



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destabilization of the 3MC state is expected to increase like 13PYAZ < 13PZAZ < 12PYAZ < 12PZAZ < 124PYAZ < 124PZAZ. The strongest π-donor among all X ligands, 13PYAZ, should create the smallest ΔO splitting (ligand field), which will result in relatively low in energy 3MC state. On the contrary, the weakest π-donor,124PZAZ, is likely to cause the highest ΔO splitting (ligand field) and to possess relatively high in energy dark state. This qualitative comparison is in agreement with experimentally estimated activation barriers for crossing from 3MLCT to 3MC excited states showing larger values for 124PZAZ (0.15 eV) than for 124PYAZ (0.07 eV).52

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10263. TD-DFT results for all S0 → Tn excitation wavelengths above 400 nm; ground state and excited state geometries obtained with the PBE0 method; MOs and CI coefficients for S0 → Tn and Tn → S0 transitions; ground state orbital energies for the isolated ligands and the corresponding complex ions; comparison between experimental and PBE0 ground state geometries; atomic coordinates (PDF)

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4. CONCLUSIONS We present a TD-DFT investigation on single and dual emission properties of six [Ru(bpy)2X]+ complex ions, where X represents 2-pyridyl or 2-pyrazine derivatives of 1,2-diazolate, 1,3-diazolate, and 1,2,4-triazolate. Three of these complex ions are known in the literature, and the other three have been proposed here. We demonstrate how the nitrogen content and position in the X ligand affect the single and dual emission wavelengths. We further show that in all complex ions the first emission wavelength is associated with 3MLCT state localized on one of the bipyridine ligands (3MLCT (bpy)), while the second potentially emitting 3MLCT state can be entirely localized on the X ligand (3MLCT (X)) or also to span part of the bipyridine ligands. All X ligands create relatively strong ligand fields, which are expected to ensure destabilization of the photodeactivating 3MC states with respect to the two 3MLCT emitting ones and to have a positive impact on the single and dual emission quantum yields. As a function of the X ligand, the first emission wavelength varies between 743 and 856 nm and the second emission wavelength changes between 555 and 679 nm. A blue shift in the first emission wavelength can be obtained by increasing the nitrogen content in the sixmembered ring of the X ligand or by going from 1,2- > 1,3> 1,2,4-azolate derivatives. A red shift in the second emission wavelength excited state can be obtained by increasing the nitrogen content in the six-membered ring of the X ligand or by using 1,2- and 1,2,4-azolate derivatives rather than 1,3-azolate derivatives. We demonstrate also that by X ligand modification the energy difference between the two possible 3MLCT emitting states can be tuned from 84 to 310 nm. The energy difference between the two states decreases when the energy difference between the LUMOs (reduction potentials) of the bipyridine and the X ligand becomes smaller. We also show that a small energy difference between the LUMOs of the isolated bipyridine and X ligand extends the delocalization in the second 3 MLCT state, which suggests an increase in the probability for electronic coupling between 3MLCT (bpy) and 3MLCT (X) excited states. If the energy difference between the ligands LUMOs is small enough, it is expected to cause deactivation of the 3MLCT (X) excitation and disappearance of the second emission peak. Therefore, our results hint that the variation in the nitrogen content and position in the X ligand can be used to tune the photodynamics in these systems and possibly to design room temperature dual emitters. Further work would be to apply the theoretically derived tunability trends in the laboratory synthesis and to test experimentally the photophysical properties of the newly designed complex ions.

AUTHOR INFORMATION

Corresponding Author

*P. D. Jarowski. E-mail: [email protected]. ORCID

Julia Romanova: 0000-0001-6668-0879 Present Address §

University of Lincoln.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors confirm that data underlying the findings are available without restriction. Details of the data and how to request access are available from the University of Surrey publications repository. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Leverhulme Trust (RPG-2014-006) for funding and the National Service for Computational Chemistry Software (NSCCS) at Imperial College London.

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ABBREVIATIONS TD, time dependent; DFT, density functional theory; PCM, polarizable continuum model of the solvent; MLCT, metal-toligand charge transfer; MO, molecular orbitals; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital

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